The light-dependent reactions
Light dependent reactions actors
- [Voiceover] In a previous video, we gave an overview of the light dependent reactions which are essentially occurring across the thyla or within or across the thylakoid membranes, right that we zoomed in on one and we saw okay we have some energy from light exciting the electrons within that chlorophyll pair, that P680 chlorophylled A pair. That electron, that energized electron will then be transferred from one molecule to another and as it does so it will go to lower and lower energy states. And that released energy, some of it will be used to transfer hydrogen protons across the membrane. And then eventually that electron will make it's way to Photo System I where it can get excited again. If we think of it as the same electron. It doesn't necessarily have to be the exact same electron. But we can think of that same electron as being excited again by light energy and then it can once again go to lower and lower energy states and this time it's going to be used to reduce NADP+ to NADPH. Now NADPH itself is an input into the Calvin Cycle. But ATP is another input we need for the Calvin Cycle and the way that we produce ATP is that hydrogen ion concentration that increases on the inside due to it being essentially pumped across the membrane, as well as the leftover hydrogen ions from the water after it's stripped of electrons, to replace that originally excited electron in that P680 chlorophyll pair. Well that increased hydrogen ion concentration can be used to drive ATP synthase which creates ATP from phosphate and ADP. And we saw it, we saw that over here, without seeing the different components. You get light, excite the electron. The electron goes to lower and lower energy states. As it does so it's going from Photo System II to Photo System I. Some of that energy is being used to pump hydrogen ions into the thylakoid lumen. Then that electron can get excited again and then as it gets transferred and goes to lower and lower energy states, it can be used to produce NADPH where once again it's electrons are still at a fairly high energy state so it's a strong reducing agent. And so that's why it's valuable in the Calvin Cycle. That energy from acting as a strong reducing agent can be used to, or help in the creation or the eventual creation of the sugar. And once again where as an electron, once it gives it away, how does it get replaced? Well it snags it from the water. What I have here is a more detailed diagram that labels some of the actors and the important thing is really what we just covered and what we covered in more detail in the previous video. The conceptual idea of what's happening in the light dependent reactions. But a lot of times in your biology class or in your biology book, you'll see talk of things like a cytochrome complex and plastoquinone and things like that and I want you to look at that right now so you're not intimidated when you see it and that you see that these are just the actors that we talked about. So right over here, this is Photo System II, and what you have, and I give credit for where this image comes from. It's modified from The Light Dependent Reactions of Photosynthesis Figure 8 by OpenStax College. But this right over here, we see the light is, the light is interacting the way it's depicted here, not directly with the chlorophyll pair within Photo System II that, that P680 chlorophyll A pair. We see it acting on some of these neighboring molecules as their electrons get excited and then go to lower energy levels, that energy can be used to excite neighboring electrons. This kind of keeps happening. That energy gets transferred eventually to excite the electron in that P680 pair and then that electron, the first electron acceptor, you'll see this sometimes spoken of in your biology textbooks, is pheophytin. And that can then transfer the electron to plastoquinone, and that plastoquinone is interacting in this cytochrome complex which transfers the electron from plastoquinone to plastocyanin and as it's doing it you see the hydrogen ions being transferred from the outside of the thylakoid to the inside of the thylakoid, which is exactly what we've been talking about. And then as we go to Photo System I. Well that electron can be transferred from the plastocyanin to the chlorophyll pair, the P700 chlorophyll. That can get excited again. Once again, it doesn't have to be the light directly exciting it. It can be exciting other molecules within the Photo System I, but that energy eventually gets transferred to that chlorophyll, excites it's electrons and then it goes from one molecule to another. Eventually goes to ferredoxin which is being used in conjunction. It's one of the actors that the enzyme NADP+ reductase needs along with NADP+. So it's essentially just reducing NADP+ along with this electron that's on the ferrodoxin to produce NADPH. And once again what's going on here? Well this is the ATP synthase that is using all this increased hydrogen ion concentration on the inside of the thylakoid to pump or to power the motor, or the ATP synthase is the motor that is powered as these hydrogen ions go down their concentration gradient. And that energy is used to jam the phosphate on to the ADP to produce ATP. So I've said essentially the same thing two or three times already in the last two or three videos but I was doing it because when you first see this it seems very very intimidating and very very complex, and it is complex, and frankly it's amazing that things like this are happening on the plant that I'm looking at outside my window right now. It boggles my mind that this kind of thing is happening in nature. And there are bits and pieces of it that aren't fully understood yet and still need to be discovered. But at the same time the general idea is not as intimidating as these diagrams appear. So hopefully you find this awe inspiring, like I do. And not as intimidating as what some of these words might make you feel initially.
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